Due to the recent proliferation of the kitchen microwave oven, "microwave" has become literally a household word. As a result of the knowledge gained by the mass production of these ovens, the per kilowatt cost of efficient microwave energy sources has fallen precipitously, opening a broad range of new applications in both commercial and industrial settings. One application is the use of microwave energy to efficiently initiate and sustain plasmas for use in plasma treatment processes, semiconductor etching, thin film deposition processes and other processes.
While the conventional microwave oven is designed to be adequate to uniformly heat food products through the use of mechanical means to average out microwave energy non-uniformities and while taking advantage of the relatively long thermal relaxation times of the food products which are being heated, the same techniques cannot be used for the task of uniformly exciting gases to create a plasma because of their short relaxation times. The fans and other mechanical "microwave dispersers" used in oven technology, are unable, regardless of how fast they can practically turn, to assure a uniform dispersion of microwave energy, on a time scale appropriate to plasma excitation. To accomplish the uniform microwave excitation of a plasma other means must be employed. Prior examples of microwave plasma deposition techniques are illustrative of the state of the art and highlight both the problems encountered in increasing the energy uniformity and the advantages provided by the novel microwave plasma generating structure of the instant invention.
Commonly assigned, U.S. Pat. Nos. 4,517,223 and 4,504,518 to Ovshinsky, et al, both entitled "METHOD OF MAKING AMORPHOUS SEMICONDUCTOR ALLOYS AND DEVICES USING MICROWAVE ENERGY", the disclosures of which are incorporated herein by reference, describe processes for the deposition of thin films onto small area substrates in a low pressure, microwave glow discharge plasma. As specifically noted in these Ovshinsky, et al patents, operation in the disclosed low pressure regimes not only eliminates powder and polymeric formations in the plasma, but also provides the most economic mode of plasma deposition. While these patents describe a truly remarkable regime of low pressure and high energy density, deposition utilizing microwave energy, i.e., at substantially the minimum of the Paschen curve, the problem of uniformity of deposition over large areas remains unaddressed.
Turning now to microwave applicators for large area substrates, commonly assigned U.S. Pat. No. 4,729,341 of Fournier, et al, for "METHOD AND APPARATUS FOR MAKING ELECTROPHOTOGRAPHIC DEVICES", the disclosure of which is incorporated by reference, describes a low pressure microwave initiated plasma process for depositing a photoconductive semiconductor thin film on a large area cylindrical substrate using a pair of radiative waveguide applicators in a high power process. However, the principles of large area deposition described therein are limited to cylindrically shaped substrates, such as electrophotographic photoreceptors, and the teachings provided therein are not directly transferable to large area, generally planar substrates.
Many workers in the field have disclosed methods of processing thin films utilizing the high power of microwave sustained plasmas. However, microwave plasmas have not been altogether appropriate for large surface area and/or low pressure deposition. This is because of the non-uniformity of the resulting surface treatment, a consequence of the non-uniformity of the energy. One attempt to provide greater uniformly was the use of a slow wave microwave structure. A problem that is inherent in slow wave structures, however is the very rapid fall off of microwave coupling into the plasma as a function of distance transverse to the microwave applicator. This problem has been addressed in the prior art by various structures that vary the spacing of the slow wave structure from the substrate to be processed. In this way the energy density at the surface of the substrate is constant along the direction of movement of the substrate. For example, U.S. Pat. No. 3,814,983 to Weissfloch, et al for "APPARATUS AND METHOD FOR PLASMA GENERATION AND MATERIAL TREATMENT WITH ELECTROMAGNETIC RADIATION" and U.S. Pat. No. 4,521,717 to Kieser, et al, for "APPARATUS FOR PRODUCING A MICROWAVE PLASMA FOR THE TREATMENT OF SUBSTRATE IN PARTICULAR FOR THE PLASMA POLYMERIZATION OF MONITORS THEREON", both address this problem by proposing various spatial relationships between the microwave applicator and the substrate to be processed.
The slow wave structure is described, for example in U.S. Pat. No. 3,814,983 to Weissfloch, et al, for "APPARATUS AND METHODS FOR PLASMA GENERATION AND MATERIAL TREATMENT WITH ELECTROMAGNETIC RADIATION" and in U.S. Pat. No. 4,521,717 to Kieser, et al, for "APPARATUS FOR PRODUCING A MICROWAVE PLASMA FOR THE TREATMENT OF SUBSTRATES, IN PARTICULAR FOR THE PLASMA-POLYMERIZATION OF MONITORS THEREON". More particularly, the Weissfloch, et al patent discloses the problems encountered in obtaining uniform field intensity. Weissfloch, et al discloses that in order to obtain the uniform electric field intensity necessary for a plasma of uniform power density along the full length of the slow wave waveguide structure, it is necessary to incline the waveguide structure at an angle with respect to the substrate. Inclination of the slow wave waveguide structure to achieve uniformity with respect to the substrate leads to an inefficient coupling of microwave energy into the plasma.
Recognizing this deficiency of the slow wave structure, Kieser, et al proposed the use of two waveguide structures in an anti-parallel arrangement. In this way the energy inputs of the two structures are superimposed one upon the other. More particularly, Kieser, et al described that the conditions resulting from superposing of two energy inputs, i.e., two microwave applicators, can be further improved if the two slow wave applicators are set at an angle to each other such that the planes normal to the medians of the applicators intersect at a straight line which extends parallel to the surfaces of the substrate to be treated and at right angles to the travel direction of the substrate. Moreover, Kieser, et al recommend that in order to avoid destructive interference of the wave field patterns of the two applicators, the applicators should be displaced from each other transversely of the travel direction of the substrate by a distance equal to half of the space between the cross-bars of the waveguide. Kieser, et al disclose that in this way the microwave field pattern is substantially suppressed.
The problem of plasma uniformity and more particularly, energy uniformity is treated by J. Asmussen and his co-workers, for example in T. Roppel, et al "LOW TEMPERATURE OXIDATION OF SILICON USING A MICROWAVE PLASMA DISC SOURCE", J. Vac. Sci. Tech. B-4 (January-February 1986) pp. 295-298 and M. Dahimene and J. Asmussen "THE PERFORMANCE OF MICROWAVE ION SOURCE IMMERSED IN A MULTICUSP STATIC MAGNETIC FIELD" J. Vac. Sci. Tech. B-4 (January-February 1986) pp. 126-130. In these as well as other papers, Asmussen and his co-workers describe a microwave reactor which they refer to as a microwave plasma disc source ("MPDS"). The plasma is reported to be in the shape of a disc or tablet, with a diameter that is a function of microwave frequency. A critical advantage claimed by Asmussen and his co-worker is that the plasma disc source is scalable with frequency, that is, at the normal microwave frequency of 2.45 gigahertz the plasma disc diameter is 10 centimeters and the plasma disc thickness is 1.5 centimeters but that the disc diameter can be increased by reducing the microwave frequency. Asmussen and his co-workers disclose that in this way the plasma geometry is scalable to large diameters, potentially yielding a uniform plasma density over a large surface area. However, Asmussen only describes a microwave plasma disc source which is designed for operation at 2.45 gigahertz, where the plasma confined diameter is 10 centimeters and the plasma volume is 118 cubic centimeters. This is far from a large surface area. Asmussen and his co-workers, however, propose a system designed for operation at the lower frequency of 915 megahertz, saying that the lower frequency source would provide a plasma diameter of approximately 40 centimeters with a plasma volume of 2000 cubic centimeters.
Asmussen and his co-workers further describe that the microwave plasma disc source can be used as a broad beam ion source or as a plasma source for material processing and can be scaled up to discharge diameters in excess of 1 meter by operating at still lower frequencies, for example 400 megahertz. The microwave plasma disc source of Asmussen and his co-workers, while, in principle, providing the relatively large surface area requires frequency adjustment to do so. There are severe economic consequences of this approach to variation of the dimensions of a plasma processing machine. Only 2.45 GHz magnetrons have been developed to be both inexpensive and to have large power capabilities. High power microwave sources at other fixed frequencies remain expensive and variable frequency high power microwave sources are extremely expensive.
Furthermore, the deposited material quality and deposition rate is dependent on excitation frequency. This changing frequency to increase plasma dimensions may entail compromises in material quality and film deposition rate. Additionally, the magnets which are used in the system disclosed by Asmussen must be made larger in size, and different in field strength as the excitation frequency is changed. Thus, as a means of changing the plasma dimensions, Asmussen's approach has the disadvantage of rigidly coupling other important deposition parameters and therefore reducing operational flexibility.
Workers at Hitachi have described, for example in U.S. Pat. No. 4,481,229 to Suzuki, et al the use of electron cyclotron resonance to obtain a high power plasma having relatively high degree of uniformity over a limited surface area. However, the Hitachi patent does not teach, nor even suggest a method by which uniform large area plasmas may be achieved. Moreover, the use of electron cyclotron resonance imposes the added requirement of an additional highly uniform magnetic field structures in the microwave apparatus, and may be restricted in operation to only those very low pressure regimes where electron collision times are long enough to allow the cyclotron resonance condition to be achieved.
U.S. Pat. Nos. 4,517,223 and 4,729,341 referred to above, describe the necessity of using very low pressures in very high microwave power density plasmas. The use of low pressures in necessary in order to obtain high deposition rates and/or high gas utilization; U.S. Pat. Nos. 4,517,223 and 4,729,341 emphasize the criticality of low plasma pressure in order to economically carry out the plasma processes. However, the relationship between high deposition rates, high gas utilization, high power density, and low pressure further limits the utility of slow wave structures and electron-cyclotron resonance methods. The limitations of the slow wave structure and of the electron-cyclotron resonance methods are obviated and the deposition rates and low pressure regimes described in the aforementioned U.S. Pat. Nos. 4,517,223 and 4,729,341 are obtained by the method and apparatus described hereinbelow.